1. Field of the Invention
The present invention relates generally to radio receivers, and in particular, to an improvement in an automatic gain controlling function of a radio receiver. More specifically, the present invention relates to an improvement in an automatic gain control system of a radio receiver comprising an automatic channel selecting function.
2. Description of the Background Art
Such an AM radio receiver as shown in FIG. 1 is well known. In FIG. 1, the radio receiver comprises an RF amplifying circuit 2 for amplifying an incoming radio frequency (RF) signal through an antenna 1, and a mixing circuit 3 for mixing the RF signal with a local oscillating signal from a local oscillating circuit 4 to output an intermediate frequency signal (IF signal) of a fixed frequency.
Local oscillating circuit 4 changes the oscillating signal frequency in response to a tuning frequency control signal VT for determining a desired channel frequency.
Mixing circuit 3 outputs the intermediate frequency signal by utilizing the beating by oscillating signal and the wide band RF signal. The intermediate frequency signal accordingly has a band width entered at a fixed frequency (for example 450 KHz). The intermediate frequency signal generated by mixing circuit 3 based on the wide band RF signal is referred to as a wide band IF signal. Mixing circuit 3 may include a first mixing circuit for generating a first intermediate frequency (IF) signal having a center frequency of, for example, 10.7 MHz from the wide band RF signal and a second mixing circuit for converting the first IF signal to a second IF signal of 450 KHz, for example. In this case, the first mixing circuit receives a variable oscillating signal and the second mixing circuit receives an oscillating signal of a fixed frequency.
The radio receiver further includes an IF amplifying circuit 5 for amplifying the IF signal from mixing circuit 3 and a detection circuit 6 for detecting the output signal of IF amplifying circuit 5 (the signal of a desired channel), to output a low frequency signal. The output of detection circuit 6 is applied to a down-stream low frequency amplifying circuit wherein it is reproduced by a speaker.
IF amplifying circuit 5 includes an IF tuning circuit. A signal passing through IF tuning circuit has its band limited. A signal at the input of IF amplifying circuit 5 is referred to as a wide band IF signal, while a signal at output thereof is referred to as a narrow band IF signal.
The radio receiver further includes a gain control (AGC) circuit 7 for compensating for a fluctuation of an incoming RF signal from antenna 1 to substantially fix the output level of RF amplifying circuit 2. AGC circuit 7 controls the gain of RF amplifying circuit 2 in response to the level of the wide band IF signal from mixing circuit 3. Mixing circuit 3 simply mixes the RF signal with an oscillating signal from a local oscillator 4, whereby the level (strength) of the wide band IF signal is proportional to a field strength of the incoming RF signal at the antenna 1. As a result, with the amplification of the incoming RF signal controlled in response to the wide band IF signal, it is possible to control gain in response to the level (reception field strength) of the incoming RF signal. The circuit is hereinafter referred to as a wide band AGC circuit which controls the gain of RF amplifying circuit 2 by using a wide band IF signal.
The wide band AGC circuit 7 decreases the gain of RF amplifying circuit 2 when the level of the wide band IF signal increases. Wide band AGC circuit 7 increases the gain of RF amplifying circuit 2 when the level of the wide band IF signal decreases.
When the level of the wide band IF signal increases, the gain of RF amplifying circuit 2 is reduced to lower the level of the wide band RF signal applied to mixing circuit 3, thereby preventing saturation of mixing circuit 3 to improve a cross modulation interference characteristic. However, wide band AGC circuit cause several problems. More specifically, when the RF signal includes a strong interference signal and a weak desired signal (desired channel frequency signal), the output of mixing circuit 3 includes a signal obtained by frequency-converting the strong interference signal. Wide band AGC circuit 7 controls the gain of RF amplifying circuit 2 in response to the strong interference signal. As a result, the desired signal component is suppressed to reduce sensitivity to the desired signal.
Some radio receivers have an automatic channel selecting function. At an automatic channel selection, a tuning frequency (the control signal VT in the arrangement of FIG. 1) is successively changed corresponding to interchannel steps. When a narrow band IF signal level (reception field strength) exceeds a predetermined reference value, the determination is made that the channel is selected.
A tuning frequency is fixed to the frequency exceeding the predetermined reference value. For automatic channel selection, a level detector is required which detects a level of a narrow band IF signal. Such a level detector exhibits an input/output characteristic shown in FIG. 2.
In FIG. 2, the abscissa denotes a strength of an incoming RF signal through antenna 1 and the ordinate denotes an output of a level detector. The curves I and II encircled by the doted lines show changes of an output V.sub.SM of the level detector when a tuning frequency is changed at an incoming RF signal levels V1 and V2, respectively.
In order to further improve the cross modulation characteristic of the radio receiver shown in FIG. 1, proposed is a radio receiver using a wide band RF signal at a channel selection which uses a narrow band RF signal for a reception after the channel selection.
This radio includes an RF tuning circuit in an RF amplifying circuit. At a channel selection, the RF amplifying circuit outputs a wide band RF signal which is applied to a mixing circuit. After the channel selection, the RF tuning circuit is tuned to a selected channel frequency. The RF tuning circuit outputs an RF signal (narrow band RF signal) tuned to the selected channel frequency from the incoming RF signal, for application to the mixing circuit 5.
This radio receiver utilizes a wide narrow band RF signal for channel selection to eliminate tracking error. For reception after the channel selection, only a narrow band RF signal, that is, a tuned RF signal, is applied to the mixing circuit to suppress cross modulation interference caused due to an interference signal component included in the wide band RF signal.
The radio receiver has the mixing circuit receiving different signals at a channel selection and at a reception after the completion of the channel selection, thereby requiring a fine tuning of a tuning frequency after the channel selection.
The fine tuning circuit tunes the RF tuning circuit to the tuning frequency selected based on the wide band RF signal at the channel selection (referred to as a rough tuning). After the rough tuning, the tuning frequency of the RF tuning circuit is changed by a predetermined step. The RF tuning circuit is set to the tuning frequency providing the maximum reception field (narrow band IF signal level).
In this case, when a signal of a field strength below 95 dB.mu. is received, sufficient change is obtained as shown by the curve II in FIG. 2. However, when a strong field signal of a field strength above or equal to 95dB.mu. is received, the change is quite small so that accurate fine tuning can not be accomplished.